Whereas passenger vehicles and light utility vehicles usually comprise a pedal force-assisted hydraulic braking system and heavier utility vehicles comprise a power-assisted pneumatic braking system, wheeled vehicles that are provided for off-road use, such as construction machinery and agricultural tractors, are equipped with a power-assisted hydraulic braking system. Because of the necessary high braking forces, the actuating units of the wheel brakes, such as the wheel brake cylinders and brake calipers, in such wheeled vehicles comprise a relatively large displacement volume, which can only be delivered with a power-assisted braking system. Because construction machinery and agricultural tractors usually comprise hydraulic working units or even a hydrostatic drive, a power-assisted hydraulic braking system can advantageously use the hydraulic pressure supply device that is already provided.
In the present case, a power-assisted hydraulic braking system of a wheeled vehicle will be assumed, comprising a main brake line and a plurality of wheel brake lines branching off from the main brake line and each leading to an actuating unit of a wheel brake and a valve arrangement of an ABS controller. A setpoint brake pressure can be introduced into the main brake line by means of a brake valve that is actuated by a driver by means of a brake pedal. With an embodiment of the wheel brakes embodied as drum brakes, the actuating units of the wheel brakes are usually embodied as wheel brake cylinders, wherein the pistons thereof press the relevant brake shoes against the brake drums that are rotationally fixedly joined to the wheel hubs to decelerate the vehicle wheels. With an embodiment of the wheel brakes as disk brakes, the actuating units of the wheel brakes are usually brake calipers, in which the relevant brake linings are pressed by means of pistons against the brake disks that are rotationally fixedly joined to the wheel hubs to decelerate the vehicle wheels.
The valve arrangement of the anti-lock brake system controller (ABS controller) comprises an inlet valve disposed between the main brake line and one of the wheel brake lines and an outlet valve disposed between the relevant wheel brake line and a pressureless return line in each case. In the non-actuated state, the inlet valves are opened and the outlet valves are closed, so that the setpoint brake pressure specified by the driver is also effective in the wheel brake lines and the actuating units of the wheel brakes that are connected to the same. Consequently, the wheeled vehicle involved is decelerated according to the operation of the brake pedal by the driver. However, if during a braking process a wheel that is locked or will shortly become locked is detected by the analysis of revolution rate sensors disposed on the vehicle wheels, the inlet valve of the relevant actuating unit is closed and the associated outlet valve is opened, so that the wheel brake of the relevant vehicle wheel is released. If said vehicle wheel is again rotating at a sufficiently high revolution rate, the inlet valve of said actuating unit is opened again and the associated outlet valve is closed until a large reduction in the revolution rate is detected on the relevant vehicle wheel. By alternately opening and closing the inlet valves and the outlet valves, locking of the vehicle wheels is thus prevented and as a result the ability to brake and steer the relevant wheeled vehicle is maintained.
In order to enable a reduction of the brake pressure via the main brake line and the brake valve in the event of inlet valves and outlet valves being closed at the same time as a result of a fault, if by releasing the brake pedal there is a higher brake pressure in one of the wheel brake lines than in the main brake line, safety valves are usually provided. Said safety valves can be implemented in the form of non-return valves, in each case disposed parallel to the inlet valves between the wheel brake lines and the main brake line and opening towards the main brake line, or in the form of non-return valves or emergency opening functions integrated within the inlet valves with an opening direction towards the main brake line.
In DE 195 46 056 A1 and EP 1 013 528 B1, power-assisted hydraulic braking systems of a motor vehicle are described, each comprising two brake circuits, a valve arrangement of an ABS controller and a valve arrangement of an anti-slip controller (ASR controller). The inlet valves and the outlet valves of the valve arrangement of the ABS controller are each embodied as 2/2-way solenoid switching valves. Whereas the safety valves for the valve arrangement of the ABS controller according to DE 195 46 056 A1 are embodied as non-return valves, each disposed parallel to the inlet valves, the valve arrangement of the ABS controller according to EP 1 013 528 B1 comprises non-return valves or non-return valve functions integrated within the inlet valves for this purpose.
Because of a regulation of the European Union, all braking systems of agricultural vehicles that reach or exceed a maximum speed of 40 km/h must comprise an ABS controller by no later than 01.01.2020. Especially for large and heavy agricultural tractors with correspondingly large-dimension wheel brakes, where they are fitted with a power-assisted hydraulic braking system, valve arrangements of the ABS controller with high volumetric throughputs and very high ABS control frequency are therefore necessary. Said requirement is hardly achievable with inlet valves and outlet valves embodied as solenoid switching valves, because either the switching cross-sections of the solenoid switching valves are too small for this or the switching dynamics of the solenoid switching valves are too low and the current drain thereof is too high.
ABS control valves are offered by MICO Inc., USA for the ABS control of heavy wheeled vehicles for off-road use, being embodied as 3/3-way proportional solenoid valves and each combining the functions of an inlet valve and of an outlet valve. Such ABS control valves are indeed configured for a high volumetric throughput, but only enable a relatively low ABS control frequency.